Telephone systems of the digital signal type require analog to digital and digital to analog converter circuits, usually referred to as CODECs for interfacing voice signals and the like between analog signals compatible with analog transducers and binary signals compatible with digital telephony switching and transmission function and apparatus. In order that the encoding function be performed such that pulse code modulated (PCM) words are generated which accurately represent an analog input signal, the analog signal must be centered about an encoder midpoint. That is, the encoder operating midpoint and the zero level of the analog signal should correspond. If not, quantization distortion, harmonic distortion and noise are included in the PCM words. It is typical practice to have predetermined the gain of an encoder during manufacture, and if desired, later when in use, alter the gain, in circuit, with the use of a preset gain pad. Biasing of the midpoint may be conveniently achieved by AC coupling an appropriate node within the encoder and charging the node via a switched capacitor resistance to null any offset at the encoder input. In effect, the sign bit of each PCM word is integrated onto the AC coupled node such that its long term voltage (d.c. value) is representative of the average of the sign bit polarities. Stability is achieved when the sign bit occurrences are evenly negative and positive.
If however the gain of the typical encoder is changed during its normal operation, a step component consequently appears at the input of the encoder because the integrated signal at the AC coupled node no longer cancels the total offset. Hence, the operational midpoint is shifted to no longer correspond to the zero level of the analog signal. In a linear encoder the effect of a gain change will be manifest as an annoying click. In a compression encoder, severe harmonic distortion may also result. Given time, the step voltage diminishes until the operational midpoint again corresponds to the zero level of the analog signal. However, during this time resulting harmonic distortions deteriorate the quality of compression encoded signal. In an application wherein the gain of the encoder is adjusted from time to time, or frequently, such performance is unacceptable.